Staphylococcus aureus Bacteremia - A Simple Guide

What Is It?

Sources - How Does It Get Into the Blood?

1. Intravenous Drug Abuse (IVDA)

• People who inject drugs repeatedly puncture their skin with non-sterile needles
• Each injection is a direct route for bacteria straight into a vein
• This is why injection drug use has become one of the leading causes of both bacteremia and endocarditis worldwide - Harrison's notes the recent "upsurge in injection drug use" as a major driver of increasing endocarditis incidence
• These patients often get right-sided endocarditis (tricuspid valve) because injected material hits the right heart first

2. Intravascular Catheters (IV lines, PICC lines, central lines)

• Any tube left in a blood vessel creates a foreign body - a perfect surface for S. aureus to stick to and form a protective biofilm
• Studies found an endocarditis rate of ~25% among patients with catheter-associated S. aureus bacteremia (Harrison's 22E)
• Catheters are the main hospital (nosocomial) source

3. Other Common Sources

How Bad Can It Get? - Complications (Seeding)

1. Heart - Infective Endocarditis (IE)

• The most feared complication
• Bacteria stick to heart valves and form vegetations (lumpy colonies of bacteria + clot)
• S. aureus is now the #1 cause of endocarditis worldwide (25-35% of all cases)
• Signs to look for: new heart murmur, fever, small skin spots (Janeway lesions, Osler's nodes), stroke, septic emboli to lungs
• Can destroy the valve rapidly (acute presentation), causing heart failure
• Right-sided IE (tricuspid valve): Classic in IV drug users - causes septic pulmonary emboli (circular lesions on chest X-ray that can cavitate)
• Left-sided IE (aortic/mitral valve): More dangerous, emboli can go to brain, kidneys, spleen

2. Bones and Joints - Osteomyelitis & Septic Arthritis

• S. aureus is the most common cause of both
• Bacteria land in bone (especially spine, long bones) or joints through the bloodstream
• Presents as: bone pain, fever, inability to move a joint
• Spinal epidural abscess can form - this is a neurosurgical emergency (spinal cord compression)
• In IV drug users, the sternoclavicular and sacroiliac joints are commonly affected

3. Kidneys

• Renal abscesses (pus pockets in the kidney) from hematogenous seeding
• Can also cause perinephric abscess (abscess around the kidney)
• Bacteriuria (bacteria in urine) can be a clue to hematogenous kidney involvement
• If a patient has S. aureus in the urine without a urinary source, think: the kidney was seeded from the blood

4. Lungs

• Septic pulmonary emboli - infected clots from right-sided IE travel to the lungs
• Appear as multiple round spots on chest X-ray, which can cavitate (hollow out) over time
• Presents as: pleuritic chest pain, hemoptysis, purulent sputum

5. Brain/CNS

• Mycotic (infected) aneurysms - bacteria weaken artery walls in the brain, which can rupture
• Embolic strokes - vegetations from the left side of the heart break off and block brain arteries
• Spinal epidural abscess - urgent back pain + neurological signs

6. Prosthetic Devices

• Any implanted hardware (pacemakers, artificial valves, hip/knee replacements) is a foreign body
• S. aureus sticks to these and forms biofilm - extremely difficult to eradicate without removing the device

Simple Visual Summary of Seeding

             S. aureus enters blood
                      ↓
    ┌─────────────────┼─────────────────┐
    ↓                 ↓                 ↓
  HEART           BONES/JOINTS       KIDNEYS
(endocarditis,   (osteomyelitis,   (renal abscess,
 IE, valve        septic arthritis,  perinephric
 destruction)     spinal abscess)    abscess)
    ↓                                  
  LUNGS           BRAIN              OTHER
(septic emboli,  (stroke, mycotic   (spleen, eyes,
 pneumonia)       aneurysm)          liver)

Management - How Is It Treated?

Pillar 1 - Source Control (Remove the "Entry Door")

• Pull the infected catheter
• Drain any abscess (pus cannot be reached by antibiotics alone)
• Remove infected prosthetics/implants when necessary
• This is the most common reason treatment fails - if the source is not removed, bacteria keep re-entering the blood

Pillar 2 - Antibiotics

Important: For MSSA, a beta-lactam (nafcillin/cefazolin) is better than vancomycin - no other antibiotic is as safe or effective for MSSA

How Long to Treat?

Criteria for "Uncomplicated" Bacteremia (Goldman-Cecil, Table 267-1)

• Fever and systemic signs resolve by day 3 of therapy
• Blood cultures turn negative within 1-2 days of starting antibiotics
• An identifiable, easily removable focus exists
• The focus is promptly removed
• No prosthetic devices, no implants
• No evidence of seeding to other organs

Echocardiography - A Must-Do

• Transesophageal echocardiography (TEE) is more sensitive than transthoracic (TTE) and should be performed to rule out endocarditis in most patients with S. aureus bacteremia
• Neither clinical nor lab findings can reliably detect cardiac involvement without it

Valve Surgery - When Needed for Endocarditis

• Heart failure from valve destruction
• Myocardial abscess
• Severe valvular damage
• Vegetation >10 mm in size

Prognosis

• Untreated S. aureus bacteremia was lethal in >85% of cases historically
• With treatment: mortality is still 20-40% depending on the patient's condition
• Methicillin resistance (MRSA) worsens outcomes partly because less effective antibiotics must be used
• Persistent bacteremia (positive blood cultures at 96 hours despite treatment) is a red flag for an undrained focus or endocarditis

Quick Memory Trick

• S - Source control first (pull catheter, drain abscess)
• E - Echo the heart (TEE to rule out endocarditis)
• E - Extended therapy (4-6 weeks for complicated cases)
• K - Know the bug (MSSA? Use nafcillin. MRSA? Use vancomycin/daptomycin)